Right, in that situation where you've not as yet made any real change towards an oldie diet (good because the longer it is till you have to, the better they're ageing), my first step is to feed cooked carbs mixed with oil. This provides longer term energy (rather than a sugar spike), is low in protein so good for kidneys, and is usually appealing to eat. As an addition to a good diet, you're looking at getting her to eat about a heaped teaspoon daily in order to maintain weight. In order to put weight on, I would be looking to feed her these extras in the early evening i.e. before you put the veg in
Examples of cooked carbs+oil would be things like:
mashed potato + veggie oil (can be normal potatoes, sweet, mix of both, additional root veggies like swede etc.)
soaked bread + coconut (I tend to melt a lump of coconut in a mug with hot water, stir it up, and pour over the bread)
cooked couscous + avocado
cooked rice + linseed oil
soaked+cooked beans + omega oil
Also if you're due a RR order anytime soon their oldies soakable mix is another option of a carby base for this.
Mix-and-match between the carbs and oil type to see what works and to keep up variety
Many older rats whose whiskers are starting to droop prefer to eat their extras from a small plate or a spoon rather than a bowl. You can also warm it up (microwave or add some hot liquid) to make it more appealing and get her to eat more at each sitting.
The above combination is also really easy to lace with supplements, or add Complan or dried baby food to so as to add fortified powders and varying the flavour too.
If/ when she gets to the point of only minimally eating her dry mix, then building on the carbs+oil to be more rounded in terms of protein is also pretty easy.